For example, in a case where a power converter is particularly used to drive a motor of an electric vehicle, the power converter needs to have high responsiveness so as to make its output follow at high speed according to change in driving condition. In such a power converter, it has been known, as effective means, to sense a current flowing in a switching element, in addition to input and output voltages, and to perform control according to the sensed current. However, sensing a conduction current of the switching element is likely to cause power loss according to the sensed current, resulting in deterioration of efficiency. Therefore, it is difficult to put such means into practice.
Conventionally, as shown in FIG. 19, it has been proposed a method of connecting a current sensing resistor Rs in series to an emitter (source) terminal of a switching element Q100 and calculating a current flowing in the switching element Q100. In this method, however, when a large current is detected, power loss at the current sensing resistor Rs100 increases, resulting in decrease in power conversion efficiency of the power conversion circuit.
In order to reduce such power loss, for example, a technique disclosed in patent literature 1 is provided. In the technique of the patent literature 1, as shown in FIG. 20, a source terminal N100 of a main switching element Q93 and a source terminal N200 of a current detection switching element Q94 are feedback-controlled to have substantially the same potential using a comparator CMP401 and a switching element Q95.
Since the source terminal N100 and the source terminal N200 are controlled to have the same potential, each of the drain-to-source voltage and the gate-to-source voltage is substantially the same voltage between the switching element Q94 and the switching element Q93. Therefore, when transistors having element characteristics are used as the switching element Q94 and the switching element Q93, a conduction current of the switching element Q93 can be estimated by sensing a conduction current of the switching element Q94.
In this case, as the switching element Q94, an element having the same conductivity type as the switching element Q93, and being a transistor with a much smaller chip area is used. Therefore, a current ratio of the switching element Q94 and the switching element Q93 can be a sufficiently large constant value K.
By this reason, a current value I1 flowing in the switching element Q93 and a load 102 can be obtained as shown in the following expression (1) by detecting a voltage Vs at both ends of a shunt resistor Rr2.
                    [                  Ex          .                                          ⁢          1                ]                                                                      I          1                =                              KI            2                    =                      K            ⁢                                          V                s                                            R                                  r                  ⁢                                                                          ⁢                  2                                                                                        (        1        )                            I1: Current flowing in Q93        I2: Current flowing in Q94        K: Current ratio        Rr2: Shunt resistance value        Vs: Sensed voltage value        
In this method, when the current I2 is measured, power loss occurs because the current I2 flows through the resistor Rr2. When the constant K is set to a sufficiently large constant value, the current I2 can be sufficiently reduced and the power loss can be reduced.
However, if the above-described technical idea is employed, the following two issues exit. Firstly, the comparator CMP401 compares the source potential of the switching element Q93 and the source potential of the switching element Q94. Because the main switching element Q93 has a relatively large current capacity, a current gradient dI1/dt increases during switching.
If the current having this large current gradient is applied to an inductance being parasitic on a wiring, high induced voltage occurs. Therefore, when the switching element Q93 and the switching element Q94 are turned on and off, the voltage at the input terminal of the comparator CMP401 connected to the switching element Q93 largely varies. Since the comparator CMP401 is a small-signal analog component, if a voltage having fluctuation greater than a power supply voltage is applied to the input terminal, the element is likely to be deteriorated.
Therefore, as shown in FIG. 21, it is considered to use both the power sources relative to the input terminal voltage, as the power source of the comparator CMP401. In this structure, even if the terminal voltage of the comparator CMP401 largely fluctuates due to parasitic inductance generated at the source of the switching element Q93, the power source voltage fluctuates following to this voltage fluctuation. Therefore, it is less likely that the input terminal of the comparator CMP401 connected to the switching element Q93 will be deteriorated.
However, since the inversion input terminal of the comparator CMP401 is connected to the source of the switching element Q94, the potential of the inversion input terminal does not follow the above-mentioned voltage fluctuation. Therefore, there is a possibility that the inversion input terminal connected to the switching element Q94 deteriorates.
This issue arises because high-impedance input terminals of the comparator CMP401 are directly connected to the sources of the switching elements Q94 and Q93. Conventionally, when large voltage fluctuation occurs at the source of the switching element Q93, an overvoltage is necessarily applied between both input terminals of the comparator CMP401, resulting in the deterioration of the element.
Secondly, the following issue arises. As shown in FIG. 22, when magnetic induction coupling occurs between two wirings, the input terminal voltage of the comparator CMP401 increases, and the element is likely to be deteriorated. Since the input terminal of the comparator CMP401 is connected to the source of the switching element Q93, the magnetic induction coupling tends to increase, as compared to the other wiring.
When the switching element Q93 is switched, a large current gradient di1/dt occurs. When the magnetic induction coupling between the wirings increases, an electric current is applied via a path shown by a solid arrow in FIG. 23 including both the input terminals of the comparator CMP401 so as to cancel the current change. However, since the input terminals of the comparator CMP401 are the high-impedance terminals, the voltage between the terminals increases, and the element of the comparator CMP401 is likely to be deteriorated.
The cause of this issue is because there is a path leading to the ground through the both input terminals of the comparator CMP401, differently from a path leading to the ground from the switching element Q93 via a load 102. In a normal operation, the input impedance of the comparator CMP401 is high, and thus the current is less likely to flow.
Since this type of the comparator CMP401 is an element for a small-signal analog circuit, the comparator CMP401 is easily damaged when the excessive voltage is applied to the input terminals. Therefore, there is a possibility that the electric current is likely to instantly flow to the input terminals of the comparator CMP201 according to the electric current generated at the time of switching of the switching element Q93. Further, there is a possibility that the element of the comparator CMP401 deteriorates.
In the technique of the patent literature 1, the elements of the current detection circuit adjacent to the main switching element Q93 is likely to be deteriorated according to the induced voltage or the induced current caused by the switching operation of the main switching element Q93. The improvement of reliability is desired to be applied to good products.